The present invention relates to a fingerprint detecting device and method and, more particularly, to a fingerprint detecting device and method of irradiating light inside a fingertip through a transparent body and detecting a fingerprint pattern from reflected light.
Generally, a fingerprint detecting device for irradiating light inside a fingertip pressed against a transparent body and detecting a fingerprint pattern of the fingertip from reflected light uses optical characteristics, as shown in FIG. 4.
FIG. 4 explains the behavior of light on the boundary surface of a transparent body. Reference numeral 1 denotes a transparent body such as an optical glass member; 1S, a surface of the transparent body 1; and 8, air. In this arrangement, light which is incident from a lower portion g of the transparent body 1 on a point b of the surface 1S at an incident angle .theta.b with respect to a vertical axis jk (jk represents a line passing points j and k) of the surface 1S of the transparent body 1 emerges into the air 8 in a direction c at a refraction angle .theta.a with respect to the vertical axis jk.
Letting nb and na be the refractive indices of the transparent body 1 and the air 8, respectively, the relationship between the incident angle .theta.b and the refraction angle .theta.a is given by EQU na.multidot.sin.theta.a=nb.multidot.sin.theta.b
on the basis of the Snell's law.
Normally, the refractive index na of the air 8 is lower than the refractive index nb of the transparent body 1 such as an optical glass member (na&lt;nb), so the refraction angle .theta.a becomes larger than the incident angle .theta.b(.theta.a&gt;.theta.b).
When the incident angle .theta.b is selected to satisfy sin.theta.a&gt;na/nb, no refraction angle .theta.a can satisfy the Snell's law. The incident light is totally reflected by the surface 1S of the transparent body 1.
The incident light for causing the total reflection, i.e., a critical angle .theta.c is given by EQU .theta.c=sin.sup.-1 (na/nb)
In FIG. 4, assume that .angle.gbk =.theta.c(=.angle.hbk), light incident from the angle region represented by gbf (hbi) is totally reflected to the transparent body 1 side along the optical path dbe (ebd). On the other hand, light incident from the angle region gbh emerges into the air 8 along the optical path abc in accordance with the Snell's law. This also applies to light incident from the air 8 side, so light emerging to the absorbed or irregularly reflected by the finger 3. Therefore, the image detection unit 5 detects the fingerprint pattern of the skin surface 3A, which has bright valley portions and dark ridge portions.
However, in such a conventional fingerprint detecting device, the detected fingerprint pattern largely changes due to a gap between the surface 1S of the transparent body 1 and the finger due to a fine three-dimensional pattern present on the ridge portions of the finger 3 or the condition (dry state or wet state due to sweat) of the finger 3, so a predetermined satisfactory fingerprint pattern cannot always be obtained.
FIGS. 6A and 6B show the contact states of the finger on the surface 1S of the transparent body 1 shown in FIG. 5. As shown in FIG. 6A, when the finger 3 is dry, a small gap 35 is formed between a ridge portion 31 of the skin surface 3A and the surface 1S of the transparent body 1 because of the fine three-dimensional pattern on the ridge portion 31. Since air is present in the gap 35, light is totally reflected even at the ridge portions 31.
On the other hand, as shown in FIG. 6B, when the finger 3 is wet due to sweat or the like, sweat or grease 36 is adhered between the ridge portions 31 of the skin surface 3A and the surface 1S of the transparent body 1. Light is absorbed by a portion wider than the initial ridge portion 31, so the light to be totally reflected at a valley portion 32 decreases.
In a certain condition of the finger 3 and, more particularly, in the dry state, the fingerprint pattern is partially omitted and disconnected. In the wet state, the ridge portions 31 and the valley portions 32 of the skin surface 3A are not clearly discriminated. For this reason, a satisfactory fingerprint pattern cannot always be obtained.
A fingerprint detecting device in which a transparent elastic member is tightly mounted on a transparent body via a coupling agent, as disclosed in Japanese Patent Laid-Open No. 7-98754, or an elastic film is formed on a transparent body to bring the three-dimensional pattern of the skin surface into tight contact with the optical reference surface (fingerprint image input surface), as disclosed in Japanese Patent Laid-Open No. 6-195450, has been proposed.
However, in these arrangements, the surface where the three-dimensional pattern of the skin surface contacts the transparent elastic optical member is defined as the optical reference surface. For this reason, when the finger is wet, and sweat is present not only at the ridge portions of the skin surface but also in gaps between the valley portions and the transparent elastic optical member, incident light is absorbed even at the valley portions due to the sweat. This makes it difficult to discriminate the valley portions from the ridge portions, so a satisfactory fingerprint pattern can hardly be obtained.